1. Field of the Invention
The present invention relates to a method by which various liquid compositions are curtain-coated onto a continuously running support in strip form (hereinafter referred to as a "web") in the manufacture of photographic films, photographic papers, magnetic recording tapes, adhesive tapes, pressuresensitive recording papers, offset printing plates, or the like.
2. Background of the Related Art
The basic technology of curtain coating is described in U.S Pat. Nos. 3,508,947 and 3,632,374. In "AIChE Winter National Meeting" (1982), S. F. Kistler disclosed the theory of curtain coating, focusing on the following three phenomena which he considered would govern the rate of application by curtain coating:
(1) incorporation of tiny air bubbles between the web and the coating solution (which phenomenon is hereinafter referred to as "air entrainment");
(2) formation of a liquid deposit along the line where the coating solution contacts the web (which phenomenon is hereinafter referred to as "heel" and is common when the coating solution flow rate is large); and
(3) the coating solution is not adequately deposited but will bounce back from the web being coated (which phenomenon is caused by "heel with air entrainment" and is hereinafter referred to as "sagging" and i also common when the coating solution flow rate is large).
According to Kistler, curtain coating is no longer possible if one or more of these phenomena occur.
Various attempts have previously been made to increase the curtain coating speed limited by the aforementioned phenomena. They include:
(1) replacing the web-entrained air layer with carbon dioxide to suppress the phenomenon of "air entrainment" (see U.S. Pat. No. 4,842,900); and
(2) applying a static electric field between the web and the coating solution, whereby the adhesion of the latter is enhanced to suppress the phenomenon of "air entrainment" (see Unexamined Published Japanese Patent Application No. 197176/1987).
In fact, however, as modern coating plants adopt application speeds of 250 m/min and higher with the curtain of coating solution flowing in higher rates, the limitation of coating speeds by "heel" and "sagging" has become a greater concern than the limitation by "air entrainment". A method that has previously been proposed for dealing with this problem is:
(3) suppressing the phenomenon of "heel" by properly adjusting the shearing viscosity between the lower and upper layers of coating solution (see Unexamined Published Japanese Patent Application No. 131549/1989).
A method of curtain coating has also been proposed for insuring that a curtain forms consistently, even if the coating solution is allowed to flow in comparatively low rates, to thereby prevent the thickness of the curtain from being unduly increased at the two lateral edges, which method is:
A multiple curtain coating including the steps of forming at least two layers of a coating solution on an inclined sliding surface, allowing the superposed layers to fall down a curved lip portion at the terminal end of the sliding surface, and permitting the resulting free-falling curtain to be deposited on a running support, wherein the angle .beta. at which the sliding surface is inclined with respect to the horizontal is 30.degree.-70.degree.. This surface at which the curved lip portion contacts the superposed layers of coating solution is a cylindrical surface having a radius of curvature of at least 40 mm. This method may be modified such that the direction in which the curtain falls down forms an angle .beta. of 120.degree.-150.degree. with respect to the direction of travel of the web at the point where the curtain is deposited on the support (see Unexamined Published Japanese Patent Application No. 51170/1989). The techniques described in Unexamined Published Japanese Patent Application No. 51170/1989 and 131549/1989 are such that the flow rate of the curtain of coating solution is in the range of 1.0-4.0 cm.sup.3 /cm.multidot.sec (the unit length of coating width being expressed in centimeters). Those techniques are effective in the specified range for forming a consistent curtain and preventing its thickness from being unduly increased at both lateral edges. However, no study has been made to determine whether they are effective in suppressing the phenomenon of "sagging" in flow rates exceeding 4 cm.sup.3 /cm.multidot.sec.